1. Field of the Invention
The present invention relates to a trans-impedance amplifier (hereafter denoted as TIA), in particular, the invention relates to a trans-impedance amplifier used in an optical receiver.
2. Related Background Art
A Japanese Patent Application published as JP-2000-174564A has disclosed a pre-amplifier that which may convert a faint photocurrent received from an optical receiving device into a voltage signal. The pre-amplifier disclosed therein includes a series circuit of an FET, a cascade FET, and a load resistor as an input stage, and outputs a drain signal of the cascade FET through a source follower. The pre-amplifier further provides a function to detect a portion of the photocurrent by the current-mirror circuit, and superposes a voltage signal converted by this current-mirror circuit on the output of the pre-amplifier. The current source in the source follower, however, is constituted by an FET whose gate is short-circuited to the source thereof.
Recent optical communication system for the long distance generally implements an optical amplifier; accordingly, the optical receiver installed in such a system is required to receive an optical signal with a large optical to signal noise ratio (OSNR). An optical noise generated during ON period becomes far greater than the noise during OFF period. FIG. 8A schematically shows atypical eye-diagram coming into the optical receiver. The optical signal, which is amplified by the optical amplifier put in the optical transmission path, accompanies with a noise N as shown in FIG. 8A. The magnitude A1 of the noise N sometimes becomes comparable to a half of the magnitude A2 of the optical signal during ON period. Even the optical transmission path excludes any optical amplifiers, the magnitude A1 of the noise becomes far greater in ON period than that A2 of OFF period. The noise in OFF period is due to electrical elements implemented in the downstream of the PD.
The PD converts the optical signal containing large noise N, the pre-amplifier converts the photocurrent to the voltage signal; finally, a linear amplifier amplifies the converted voltage signal. Because the accumulative gain of the pre-amplifier and the linear amplifier sometimes exceeds 40 dB, which becomes hard to compensate a drift of DC operating conditions, the AC coupling is put between the pre-amplifier and the liner amplifier. When the cross point of the linear amplifier is set to be 0 mV, the noise N during ON period remarkably appears in the output of the linear amplifier.
To suppress the influence of the noise N, one technique has been well known, in which the input threshold of the linear amplifier is negatively shifted below 0 mV, namely to the OFF state of the optical signal. Such a technique may effectively suppress the magnitude A1 of the noise N; but, when this mechanism is to be applied to the linear amplifier, an additional circuit is necessary to control the input threshold of the linear amplifier, which is often called as an offset controller. Generally, the offset controller has a relatively complex arrangement. Accordingly, a conventional optical receiver shifts the cross point thereof to a level half of the amplitude A2 of the optical signal to enhance the OSNR.
One technique to shift the cross point of the TIA has been known where the gain or the trans-impedance is varied depending on the magnitude of the photocurrent input therein because the photocurrent becomes large in ON state of the optical signal, that is, the TIA has the non-linear gain or the non-linear trans-impedance. Shifting the cross point of the TIA over 50% of the magnitude A2 of the optical signal to reduce the gain/trans-impedance during ON state, the noise distribution of the HIGH level, which extends to a center portion of the eye diagram, may be compressed and close to the HIGH level and the noise distribution in the center portion may be vanished, as shown in FIG. 8A. Moreover, this technique may maintain the cross point in substantially center of the eye diagram, namely, close to 0 mV. Thus, the variable gain/trans-impedance technique may stably recover the data contained in noisy optical signal without controlling the offset of the linear amplifier.
However, the technique above described has an inherent subject that the shift amount in the cross point thereof depends on variation of the power supply voltage. The present invention is to provide a solution for this subject, that is, the shift amount may be substantially independent of the variation of the power supply voltage.